In a vehicle ETC system, the engine throttle is mechanically de-coupled from the driver operated accelerator pedal, and instead is positioned by an electric motor under the control of an electronic control module (ECM). The motor is activated to position the throttle in response to accelerator pedal movement, but can also be controlled to achieve other functions such as idle speed control, engine speed governing, cruise control, torque reduction for traction control, and vehicle acceleration governing. In general, the ECM or another controller determines a desired effective throttle area to achieve a given function, and the ECM activates the motor to move the throttle to a position corresponding to the desired throttle area.
The present invention concerns an improved method of vehicle acceleration governing in an ETC system. The acceleration governing function is typically requested under certain failure mode conditions, and operates under such conditions to limit the vehicle acceleration to a threshold value, which may be determined based on vehicle speed. In conventional systems, this involves a proportional-plus-integral (PI) closed-loop control which develops a throttle area command for driving the measured vehicle acceleration into correspondence with the threshold value. However, the throttle area required to maintain a given vehicle acceleration tends to increase exponentially with increasing vehicle speed. This makes the conventional proportional and integral closed-loop terms work harder to regulate the vehicle acceleration, tending to result in instability at low vehicle speeds and excessive limiting at high vehicle speeds.